The use of self locking fasteners has increased considerably in recent years for a variety of fastening applications wherein the integrity of the fastened members is critical. One common form of providing a self locking fastener is by the use of a thermoplastic resin coating which is applied to portions of the threaded bolt end or internal threads of a nut. Various methods have been developed for applying the thermoplastic material to the threaded area of the fastener. One type of method uses a patch of a soft pliable thermoplastic material which is pressed or melted into the threaded area. Some examples of this method and resulting fastener are shown in U.S. Pat. Nos. 3,093,177; 3,263,726; 3,294,139; 3,313,017; 3,416,492; 3,437,541; 3,552,467; 3,568,746; and 3,634,577. Another common method is to heat the bolt and then direct a spray of powdered resin against the heated bolt whereby the resin will melt and adhere to the threaded areas of the bolt. Examples of such coating methods are shown in U.S. Pat. Nos. 3,766,584; 3,731,724; 3,579,684; 3,787,222 and 4,120,993. In another method, the fastener has a wetting agent applied over a predetermined area and the powdered thermoplastic resin applied to the wetted area after which the fastener is heated to bond the resin to the bolt.
Although many of these known methods provide a satisfactory procedure for applying the thermoplastic resin material to the bolt, one of the disadvantages is that it is difficult to regulate the size of the circumferential area covered by the resin in a simple and inexpensive manner, and in particular to form a resin coated fastener wherein the coating is applied over a circumferential area of between 180 and 360 degrees instead of the 180 degrees area provided by as most known spraying operations. The application of the resin by spraying of the powdered material is believed to be the most economical and satisfactory but does not provide for the satisfactory coating of a circumferential area of the bolt greater than 180 degrees.
U.S. Pat. No. 4,428,981 shows a method for applying a resin coating covering approximately 360 degrees about a bolt by spinning the bolt as it passes through the coating or spray area. Although this method provides a bolt having a resin coating of more than 180 degrees, it requires a dual drive mechanism for spinning the bolts. An alternative means of coating the bolt throughout a full 360 degrees is by the use of a pair of spray nozzles, one on each side of the moving bolt to attempt to provide the 360 degree coating.
Another problem when applying a resin coating to a threaded fastener is that it is difficult to achieve a relatively thick coating of the resin on the fastener without the use of multiple spray nozzles since it is difficult to achieve a sufficient buildup of the resin particles with only a single nozzle.
Therefore, the need has existed for an improved method for coating fasteners and in particular bolts, with a self locking thermoplastic resin in which the coating can be applied throughout a predetermined circumferential area of between 180 degrees and 360 degrees of the threaded area in an extremely simple, efficient and economical procedure.